Numerous methods and devices for determining at least one flow property of fluid media, thus, of liquids and/or gases, are conventional. The flow properties as possible parameter may be any measurable physical and/or chemical properties which qualify or quantify a flow of the fluid medium. In particular, it may be a flow velocity and/or a mass flow and/or a volumetric flow.
The present invention is described hereinafter particularly with reference to what are referred to as hot-film air-mass meters as described, for example, in Konrad Reif (editor): “Sensoren im Kraftfahrzeug” (Sensors in the Motor Vehicle), 1st edition, 2010, pages 146-148. Generally, such hot-film air-mass meters are based on a sensor chip, especially a silicon sensor chip, e.g., having a sensor membrane as measuring surface or sensor area which is able to be overflowed by the flowing fluid medium. The sensor chip usually includes at least one heating element as well as at least two temperature sensors which, for example, are disposed on the measuring surface of the sensor chip, the one temperature sensor being mounted upstream of the heating element and the other temperature sensor being mounted downstream of the heating element. A mass flow and/or volumetric flow of the fluid medium may be inferred from an asymmetry of the temperature profile detected by the temperature sensors, which is influenced by the flow of the fluid medium.
Hot-film air-mass meters usually take the form of plug-in sensors, which are permanently or exchangeably insertable into a flow pipe. For example, this flow pipe may be an induction tract of an internal combustion engine.
In this context, a partial flow of the medium flows through at least one main channel provided in the hot-film air-mass meter. A bypass channel is formed between the inlet and the outlet of the main channel. In particular, the bypass channel is formed in such a way that it has a curved section for redirecting the partial flow of the medium that has entered through the inlet of the main channel, the curved section transitioning in further course into a section in which the sensor chip is located. The last-named section represents the actual measuring channel in which the sensor chip is disposed.
In practice, hot-film air-mass meters of this kind must satisfy a multitude of requirements. Besides the goal of reducing a pressure drop at the hot-film air-mass meter overall by suitable designs in terms of fluid mechanics, one of the main challenges is to further improve the signal quality as well as the robustness of the devices with respect to contamination by oil droplets and water droplets as well as soot particles, dust particles and other solid-body particles. For example, this signal quality relates to a mass flow of the medium through the measuring channel leading to the sensor chip, as well as possibly to the reduction of a signal drift and the improvement of the signal-to-noise ratio. The signal drift refers to the deviation, for example, of the mass flow of the medium in the sense of a change in the characteristic-curve relation between the mass flow actually occurring and the signal to be output, ascertained in the course of calibration during manufacture. The sensor signals output in rapid temporal sequence are considered in the ascertainment of the signal-to-noise ratio, whereas the characteristic-curve drift or signal drift relates to a change in the average value.
In the case of conventional hot-film air-mass meters of the type described, generally, a sensor carrier having a sensor chip mounted on it or incorporated into it projects into the measuring channel. For example, the sensor chip may be glued into or onto the sensor carrier. The sensor carrier together, for instance, with a metal bottom plate on which electronics, a control and evaluation circuit (e.g., having a circuit carrier, especially a printed circuit board) may also be glued, are able to form one unit. For instance, the sensor carrier may be realized as an injection-molded plastic part of an electronic module. The sensor chip and the control and evaluation circuit may be joined to each other by bonds, for example. The electronic module thus obtained may be glued into a sensor housing, for instance, and the entire plug-in sensor may be closed with covers.
German Patent No. DE 198 15 654 A1 describes a measuring device for measuring the mass of a medium flowing in a line. The measuring device has a measuring element that is circumflowed by the flowing medium and is situated in a measuring-device flow channel provided in the line. The flow channel extends along a main flow direction between an inlet opening connected to the line and at least one outlet opening downstream of the inlet opening and leading into the line. The flow channel branches out at a first separation point, located between the inlet opening and the measuring element, into a measuring channel in which the measuring element is situated, and a bypass channel that bypasses the measuring element in the main flow direction.
European Patent No. EP 0 369 592 A2 describes a measuring device for measuring the mass of a flowing fluid medium. The measuring device has a flow channel which connects an inlet opening to an outlet opening. The flow channel branches out into several further channels, one of which is the actual measuring channel in which the sensor element is disposed.
The present-day sensor systems usually have a one-sided outlet from the main channel through which particles and water or other liquids such as oil, for example, are able to be eliminated again owing to their inertia.
In spite of the numerous advantages of the conventional methods and devices, they still include potential for improvement with regard to functional aspects. Thus, the one-sided outlet necessitates that the plug-in sensor be circumflowed asymmetrically. As a result, the sensitivity to changing incident-flow conditions, e.g., due to clogging filter mats, is increased. Such changes in the characteristic curve are perceived by the control device as drift, so that in the worst case, the hot-film air-mass meter is diagnosed as defective even though the cause for the deviation lies in the altered incident flow and, for example, a filter change would solve the actual problem.
In addition, an opening for the discharge of dirt or, in the event the lateral dirt outlet is shifted into the cover, a ramp for leading to the opening may be provided in the housing body of the sensor housing. Therefore, changes in the bypass cover are only possible in so far as the position of this opening or ramp remains the same. For maximum freedom in the design of the bypass cover, it would be advantageous if the housing body would have no opening or ramp, thus, were essentially just in the area of the bypass cover. This would also mean a significant simplification in the injection-molding process of the housing body.